What is genetic screening?

Genetic screening is a health measure that involves mandatory or voluntary testing of individuals, couples, or pregnancies for a genetic condition. Genetic screening can be performed at birth, prior to conception, or during a pregnancy. Genetic screening differs from other types of genetic testing because it is offered to all individuals in a particular population even if they do not have a family history of the genetic condition. The number of genetic screening tests has grown tremendously in the past decade. With the increasing availability of genetic screening options, both providers and patients have faced many ethical dilemmas.

The most widespread use of genetic screening is the testing of newborn babies. The purpose of newborn screening is to provide immediate treatment after birth to affected infants so that the symptoms of a disease can be lessened or prevented.

Screening for phenylketonuria (PKU) began in the 1960s and is one of the oldest and best-known newborn screening programs. Blood samples are taken from the heels of newborn babies in the hospital nursery,...

Newborn Screening

The most widespread use of genetic screening is the testing of newborn babies. The purpose of newborn screening is to provide immediate treatment after birth to affected infants so that the symptoms of a disease can be lessened or prevented.

Screening for phenylketonuria (PKU) began in the 1960s and is one of the oldest and best-known newborn screening programs. Blood samples are taken from the heels of newborn babies in the hospital nursery, placed on filter papers as dried spots, and sent off to appropriate laboratories for analysis. Newborns with elevated phenylalanine levels can be effectively treated with a diet low in phenylalanine (low-protein foods). If treatment is not initiated within the first two months of life, mental retardation will occur. Individuals with PKU lack the enzyme phenylalanine hydroxylase (PAH), which converts the essential amino acid phenylalanine into the amino acid tyrosine. The lack of the enzyme PAH leads to the accumulation of phenylalanine in the body, which causes irreversible brain damage.

In addition to PKU, the newborn screen can test for other metabolic disorders, endocrine disorders (such as congenital hypothyroidism),
blood conditions, deafness, and some acquired perinatal infections. In the United States, differences exist between states in terms of what conditions are screened for on the newborn panel. In 2005, the median number of tests on the newborn screen in each state was twenty-two. Only screening for PKU and congenital hypothyroidism is mandatory in all states.

Carrier Screening

Carrier screening is the voluntary testing of healthy individuals of reproductive age who may be carriers for an autosomal recessive disorder. Autosomal recessive disorders occur when an individual inherits a nonworking gene, or mutation, from both of their parents. The parents are called “carriers” because they have one working copy of the gene and one nonworking copy of the gene. Carriers do not exhibit any symptoms of the genetic condition. However, with each pregnancy, two carrier parents have a 25 percent chance for the offspring to inherit the genetic condition.

The risk of being a carrier for an autosomal recessive disorder is often dependent upon one’s ancestry. For example, individuals of African descent have an increased risk of being a carrier for sickle-cell anemia, which is a blood disorder associated with a change in the shape of the red blood cells that can lead to difficulty transporting oxygen around the body. Individuals of Ashkenazi Jewish heritage are at increased risk of being carriers of at least ten genetic conditions. Tay-Sachs disease, which is a progressive neurological condition associated with death in infancy, is one of the best-known conditions for which Ashkenazi Jewish individuals are offered screening.

Historically, people were not always given a choice to have carrier screening. In the early 1970s, mandatory, large-scale screening of African American couples and some schoolchildren was implemented in an effort to identify carriers of the gene for sickle-cell anemia. Screening results were not kept in strictest confidence; consequently, many healthy African Americans who were carriers for sickle-cell disease were stigmatized and discriminated against in terms of employment and insurance coverage. There were also charges of racial discrimination because carriers were advised against bearing children. The laws mandating screening were later repealed. Today, carrier screening programs are very different from newborn screening programs because individuals are able to choose whether they want testing.

The choice to have carrier screening is a personal one. If both parents are found to be carriers of the same genetic condition, during a pregnancy the family is offered prenatal diagnosis via amniocentesis or chorionic villus sampling (CVS). Both procedures carry a small risk of miscarriage. Some families elect to have prenatal diagnosis so that they can prepare for the birth of a child with a medical condition. Other families may consider adoption or termination of the pregnancy if the fetus is found to have a genetic condition. Some families prefer to find out about such conditions at birth. If a couple learns that they are both carriers of a genetic condition prior to pregnancy, then their options include conceiving a pregnancy and considering prenatal diagnosis, egg or sperm donation, adoption, no pregnancy, or a fairly new technique called preimplantation genetic diagnosis. Religion, socioeconomic status, and emotions all play a role in these decisions. Genetic counselors often meet with individuals to help them decide if they want testing.

Prenatal Screening

All pregnant women are routinely offered screening tests for chromosome abnormalities such as Down syndrome, the most common chromosome condition. Individuals with Down syndrome have an extra copy of chromosome number 21 that leads to a distinctive appearance, mild-to-moderate mental retardation, and sometimes other medical issues such as heart defects or digestive system problems. The risk of having a baby with Down syndrome increases with a woman’s age, but all women have some risk. Blood and ultrasound tests are routinely offered to all women to determine if the pregnancy is at increased risk for Down syndrome. Women in the high-risk category are offered diagnostic testing such as a CVS or amniocentesis.

Impact and Applications

With the completion of the Human Genome Project, the number of genetic screening options has grown exponentially. In 2003, the American College of Obstetricians and Gynecologists recommended that providers offer all couples who are pregnant or planning a pregnancy carrier screening for cystic fibrosis, an autosomal recessive multisystem disorder that can affect the lungs, digestive system, and urogenital tract. Most states now also offer newborn screening for cystic fibrosis. In 2008, the American College of Medical Genetics issued a practice guideline stating that providers should offer all couples carrier screening for spinal muscular atrophy, an autosomal recessive neurological disorder. Some experts are advocates for population-based carrier screening for fragile X syndrome, a relatively common genetic form of mental retardation in males that can be carried by females and inherited by their sons. The technology to detect fetal cells in the maternal bloodstream is rapidly evolving, and soon pregnant woman may be able to learn if their fetus has Down syndrome with a simple blood draw.

As new tests are added to routine screening protocols and further tests are considered for population screening, society is faced with the ethical dilemma of deciding what makes a disease a candidate for genetic screening. In order for a disease to be considered for a population screening program, certain factors must exist. Some are concrete entities, such as a reliable test, infrastructure to carry out a screening program, and a high frequency of the particular disorder. Other factors are more subjective, such as the definition of the disease as “serious.”

The source of contention is that the population differs on what makes a disease “serious.” For example, many individuals involved with the Down syndrome community are opposed to the idea of offering prenatal screening because they do not see individuals with Down syndrome as very different from the rest of the population. Individuals with Down syndrome can go to school, participate in hobbies, and have meaningful interactions with their families. Similarly, some individuals who have cystic fibrosis do not see the disease as an impediment to accomplishing their life goals.

As the technology for genetic testing improves, medical professionals and lay people will both be confronted with even more ethical dilemmas about genetic screening. Where does one draw the line on what defines a disease? Is a disease simply a variation thought to be undesirable by the majority of the population? These questions are quickly becoming real issues for society to reckon with rather than something characters deal with in the world of science fiction.

Key termsamniocentesis
:
invasive procedure performed during the second trimester of pregnancy that involves the removal of a small amount of amniotic fluid with a needle to perform genetic testing on cells from the fetus
chorionic villus sampling
:
invasive procedure performed during the first trimester of pregnancy that involves the removal of a small amount of the tissue that will form the placenta for genetic testing
genetic counselor
:
professional trained in genetics and counseling who provides individuals with information about genetic testing and facilitates decision making
preimplantation genetic diagnosis
:
in this process, embryos are conceived via in vitro fertilization, and genetic testing for a particular condition is performed on the embryos prior to implantation in the uterus; only unaffected embryos are implanted
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